I always thought "vaporlock" referred to a situation where a portion of some LIQUID fuel boils into the vapor (gaseous) state,
thus thus interfering with the normal flow of the liquid. Can't imagine what that would have to do with a gas appliance.
I think it would be normal to briefly smell gas when the appliance fires up, and also normal for a small amount of residual
gas to be present after applicance shutdown. Electronic gas sniffers can detect gas in amounts that are in no way dangerous,
and could even be said to be normal. Here's a PDF document from Honeywell giving their viewpoint on the matter:
http://hphaa.com/knowledge/FAQs/gas-valve-leakage.pdf
On the other hand, if you're getting whiffs of gas every time you walk by the water heater, traceable to the gas control valve, that would
indicate to me a defective valve. It all boils down to the question: "How small is small?".
RESPONDING TO GAS LEAKAGE
COMPLAINTS
To respond effectively to gas leakage complaints,
service technicians need to take a two-step approach
before identifying the gas valve as the problem:
First, they must identify the presence of gas by
using a reliable gas detector. And second, they must
verify that the flow of gas coming from the valve is
unacceptable by using a reliable flow meter.
If gas is present in low concentrations as the
result of furnace design and the trace leakage
phenomenon, replacing a gas valve will not solve the
problem. They must educate the appliance owner
about the source of the gas odor and assure them that
it is both safe and normal for that appliance.